Geology Reference
In-Depth Information
product of the highest possible yield. The EOC, on the other hand, has
to understand the chemistry and reactions of an organic molecule but
under earth's near-surface conditions, with varying temperature (
50 1C
to
รพ
50 1C) and atmospheric pressures (950 to 41000 hectoPa (hPa)). In
addition, the laboratory analyst in a chemical company, has the job of
qualifying the purity of products to some standard specified by the
customer. The environmental chemist however, has the difficult task of
extracting, purifying and quantifying the same chemicals, their un-
wanted by-products (and possibly their degradates) from environmental
media (e.g. soil, water, biota, etc.) to a high degree of precision and
accuracy. Measuring specific chemicals in different environmental media
and understanding how the chemistry in these media 'fits together'
presents a considerable challenge, and often results in the environmental
chemist returning to the laboratory to simulate a particular process.
This chapter will explore aspects of EOC in relation to the environ-
mental behaviour and fate of anthropogenic pollutants and will explore
the physical and chemical processes that result in their environmental
partitioning and degradation; specifically photodegradation for the
latter. In each case practical examples and measurement techniques will
be presented and illustrated.
6.2 THE DIVERSITY OF ORGANIC COMPOUNDS
The wide array of organic compounds subject to research within EOC,
encompass a broad range of physical-chemical properties, but can be
categorised according to either molecular weight, volatility and/or
reactivity. Figure 1 provides examples of a range of molecules that have
vapour pressures spanning orders of magnitude, and are subject to
research because of their interesting chemistry either in the atmosphere,
aquatic systems and/or soil and sediments.
Many of the more volatile compounds (C
1
C
6
) emitted into the atmos-
phere have an important impact on atmospheric photochemical processes.
Oxidation of short-chain alkanes largely through reaction with the OH
radical, form short-lived peroxy (HO
2
.
) and alkoxy radicals (RO
2
.) that are
important for converting NO to NO
2
in the polluted atmosphere and hence
allowing the build up of ground-level ozone (O
3
) during warm sunny
weather (see Chapter 2). For higher molecular weight compounds (ZC
5
)
the isomerisation of alkoxy radical intermediates gives rise to hydroxyl
carbonyl products containing
OH,
COOH and
CQO groups. These
products comprise the major fractions of organic aerosol, and a growing
body of research literature is dedicated to understanding the oxidation
products and reaction mechanisms of vapour hydrocarbons to less volatile,
